A refrigerator is a system that lowers the temperature of a space or substance and then maintains that lower temperature. Currently, the predominant type of refrigeration system is the vapor-compression refrigeration system. Although other types of refrigeration systems such as absorption and jet ejector refrigeration systems have been developed, due to a lower coefficient of performance (COP), their uses are limited to certain applications where thermal energy is abundant. However, it is well known that many refrigerants currently used in vapor-compression refrigeration systems may have a negative environmental impact related to ozone layer depletion and global warming.
A refrigeration system using a gas as the working fluid without the change of phase is called the gas refrigeration system and operates on a gas refrigeration cycle. Regardless of a closed-loop configuration or an open-loop configuration, a gas refrigeration cycle consists of three essential processes. The first process is the compression process in which the intake working fluid is compressed to a higher temperature normally above the ambient temperature. The second process is called heat removal process in which an amount of heat is transferred from the working fluid normally to the ambient because of its above-ambient temperature after the compression to reduce its temperature. The third process is the expansion process in which the work fluid returns an amount of work to the system during the expansion and at the same time further lowers its temperature normally to a temperature below the ambient temperature as the cooled working fluid output. For a closed-loop configuration, the cooled working fluid is directed to a heat exchanger to receive an amount of heat from a space or an object being cooled. After the heat absorption, the working fluid returns to the inlet of a compressor as the intake working fluid. For an open-loop configuration, the cooled working fluid may be directly discharged into the space or object being cooled, and the intake working fluid of the compressor may be the air from the space being cooled or directly from the ambient.
More specifically, a refrigeration system using air as the working fluid may be called an air cycle machine (ACM), and operates on a Brayton refrigeration cycle that is the reverse of a Brayton power cycle. As a typical application, an ACM may be used as an environmental control system in pressurized turbine-powered aircraft. Because of the gaseous working fluid, no condensation or evaporation is involved, and the cooled air output from the process is used directly for cabin ventilation. The usual compression and expansion in a vapor-compression cycle may be accomplished in the ACM by a centrifugal compressor and an expansion turbine. In a typical configuration, system bleed air, which may be in excess of 150° C., is directed into a first heat exchanger before going through the compressor. Once the hot air is cooled, it is then compressed and sent to a second heat exchanger. The air is again cooled in the second heat exchanger and travels through the turbine which expands the air and reduces its temperature to a sufficiently low level. The work extracted by the turbine is transmitted by a shaft to the compressor to provide part of the work needed to drive the compressor.
Although the gas refrigeration system mentioned above has found some applications in niche markets, large-scale commercialization has not happened so far due to its low coefficient of performance, bulky size, and high costs. The linkage between the turbine and the compressor to transmit expansion work from the turbine to the compressor may also represent a challenge.